Morne wrote:NOTE - Stainless steel parts that get EITHER carburized OR nitrocarburized are, metallurgically speaking, no longer stainless. The reason is because the carbon combines with the chromium to form chromium carbides. This decreases from the available "free" chromium content needed to form the tightly adherent chromium oxide film. Lacking that, the steel becomes just a very expensive low-alloy steel. Why firearms makers bother to make parts from stainless steel and then nitrocarburize them is completely beyond me. They'd get virtually identical performance from a low-alloy steel like 4140 that got nitrocarburized and it would cost a LOT less. For you welders out there - this same phenomenon can happen when welding certain grades of stainless steel. That's why there are "L" grades of the 300-series stainless steels, the "L" indicates low carbon content so as to avoid making the weld-zone non-stainless.

I agree that a gun maker like S&W has no purpose in using stainless for their slide if they know they want to harden and blacken the surface with a nitrocarburized treatment. They could have used 4140 (by the way, anyone know the type of steel Glock uses?)

So certainly there is no upside to stainless except maybe marketing purposes. And it seems true that 'it is no longer stainless' on the surface. But there is another claim out there that stainless loses its corrosion resistance once nitrocarburized. From what I can gather, that actually depends on the type of stainless.

Austenitic stainless (304) corrodes more easily after being nitrocarburized in nearly all cases.

That said, 4140 which has been nitrocarburized will do about 400 hours of salt spray while 416 stainless will be about 80 with or without some type of nitrocarburization or up to 150 with other types. So once again, why not use 4140 to begin with?

You make some excellent points. 4140 is near and dear to my heart as an easy to use, rather inexpensive, low-alloy steel.

I would caution against viewing salt spray results as being indicative of real world corrosion performance. The original ASTM B-117 salt spray test was only designed to test the quality of chemical (usually chromate) conversion coatings with the failure standard being "white rust" as opposed to what your links refer to "red rust" or general corrosion of the basis metal.

I've been involved in a few coating R&D projects and have seen ASTM B-117 results utterly contradicted by field experience. Specifically, seawater (which is much more complex than simple neutral salt water due to the presence of certain minerals and microorganisms) can permit some coatings with moderate ASTM B-117 ratings to survive almost indefinitely while others get devoured. Also, when you inject Sulfur Dioxide into the salt spray chamber some of the ratings can change (including some coatings that were worse now appearing better than their peers).

Modelling accelerated corrosion with a standard laboratory test of any kind is tricky business.

trebor wrote:Regardless if it is the best choice, do you agree that 416 and 17-4 have no lower corrosion resistance after being properly nitrocarburized as they did before the process?

Good question. In fact, the links were to a relatively recent patent for salt bath nitriding of stainless through the use of a molten cyanate salt, which is different from the classical molten cyanide salt treatment. Essentially, if you put a part in this new bath at the right temperature you should only get Nitrogen diffusing inward (the balance of the cyanate gassing off as Carbon Monoxide). Thus, we're talking about nitriding here, not nitrocarburizing. Note the title on the patent:

Low temperature nitriding salt and method of use

So, the more interesting question is why does one get better and the other worse? Well, let's review:

The 304 actually is unaffected at 750 F and only minimally degraded at 850 F. All treatments higher than that absolutely ravaged it.

The 416 actually is unaffected by the shorter time treatments but is improved with longer time treatments. Note that no corrosion data is given for the 750 F and 850 F treatments at all. Interestingly, the longer time at 950 F seems to give the best result.

So, what is happening here? In both cases temperatures below 1000 F seem to be the best improving or the least damaging. Higher temperatures seem to help less or actually make the problem worse. What is it about roughly 1000 F that changes the game? The answer lies in the cyanate anion's decomposition products.

When cyanate breaks down it forms Nitrogen and Carbon Monoxide. The nitrogen rapidly diffuses into the steel but the Carbon Monoxide does not. However, the Carbon Monoxide molecule can itself be decomposed. In fact, its decomposition occurs at roughly 550 C (1022 F) when CATALYZED by the presenc of iron (the number one ingredient in all steel, stainless included). It decomposes to form Carbon Dioxide (which gasses off) and solid Carbon, which promptly diffuses into the part.

So, this method at low temperatures is JUST a nitriding treatment. At higher temperatues it also does a bit of unintentional CARBURIZING. It is still a far cry in terms of carbon input when compared to carburizing (or carbonitriding), but it does occur.

But why should two different grades of stainless react to small amounts of carburizing so differently? The key lies in the crystal structure. The 304 is an austenitic (face-centered cubic crystalline structure) steel and as such the carbon is very soluble in it. As the carbon comes in it nicely dissolves. Meanwhile, the carbon already present gets all mobile due to the thermal energy being poured in. That carbon starts tying up the chromium alloying content as chromium carbides and voila, no more stainless steel (or at least greatly reduced corrosion resistance). This is EXACTLY why if you intend to weld 304 it is SO IMPORTANT to instead use 304L (the "L" indicates very low carbon content) so as to prevent weld area sensitization. Basically, heating up an austenitic stainless steel that has carbon in it ruins it. In the case of welding it only happens in the heat affected zone. In the case of a part submerged in a molten salt bath it happens to the whole bloody part.

On the other hand, 416 is a martensitic stainless steel. In point of fact, it likely never changes crystal structure at any of the reported temperatures and thus remains as martensite (body-centered tetragonal crystalline structure) throughout the processing. The key here is that Carbon is very much NOT soluble in martensite (martensite itself being a form of highly distorted/stressed ferrite due to the trapped carbon trying to precipitate out). Thus, any Carbon that might try to diffuse in has to go interstitial and form iron carbides with the small amount of free ferrite running around. This strengthens the surface (which mechanically aids corrosion resistance by keeping down the "rust bubbling" effect) without allowing a substantial amount of the dissolved chromium to get poisoned too much. Thus, 416 pretty much remains corrosion resistant chemically and picks up some mechanical help, too. Since the mechanical help is directly proportional to case depth, longer times help more. Longer times at temperatures just below where Carbon Monoxide can decompose help most of all.

You really should come down here to UC's CAS and teach their Material Science courses instead of the current lackie. I picked up on what you were trying to say in one try, while the current joker can't decide if something is FCC or BCC half of the time - and he's finally given us back our first lab reports 6 weeks after we handed them in

We've got an ok little lab that can do basic heat treatments (no oil quenching though ), I'm sure you could come up with some interesting labs for people to do

You really should come down here to UC's CAS and teach their Material Science courses instead of the current lackie. I picked up on what you were trying to say in one try, while the current joker can't decide if something is FCC or BCC half of the time - and he's finally given us back our first lab reports 6 weeks after we handed them in

Jeep,

Thanks for your kind words. I am supposedly unqualified to teach at the college level since I do not have any funky letters after my name (unless you count B.E. or EIT, which I never sign with anyway).

Still, I do tag-team teach practical metallurgy at local businesses with a Ph.D. buddy of mine who is setting up the corrosion engineering program at the University of Akron. I'm the young, spunky high-energy guy - he's the crusty old salt with all the knowledge. He gives a good presentation on Huey Helicopter transmission failures during Vietnam (from back when he was a Metallurgist for Uncle Sam). My best spiel is on the Shuttle Columbia failure.

The beautiful thing about teaching PRACTICAL metallurgy in a business setting is that I just skip over or ignore all the high-fallutin' super-academic garbage about things like lattice vectors and whatnot. Now, if I ever teach a course to some x-ray spectrographers I'll cover that material, but your average mechanical engineer could care less. They want to know the basics of why something corrodes/breaks/cracks/bends/creeps/whatever.

Besides, I think our family name is still blacklisted at UC. My dad started his (12-year) college career there with a minor in ethanol consumption. They asked him not to come back.

We've got an ok little lab that can do basic heat treatments (no oil quenching though ), I'm sure you could come up with some interesting labs for people to do

Oil quenching is good sport. I always enjoyed showing the engineering co-ops at my old workplace the quench cycle. Poor kids spent all day squinting over machining tool paths and such, only to see a glowing orange part get plunged into the quench tank and an eruption of flame that would make the devil's BBQ look for more lighter fluid.

But yeah, even without quenching there's fun stuff. Make some intentional decarburization and show the change in surface hardness. Put some aluminum or lead on the part and demonstrate liquid metal embrittlement. With copper plating you can demonstrate all kinds of stuff like case-hardening stop-off.

Not sure if someone mentioned this... but what category would Sig's "Ilaflon" coating fall under? I've tried doing research to see the differences between Ilaflon and regular bluing, but can't find much. My Sig Pro SP2022 has Ilaflon and I love how it looks

"Opportunity is missed by most people because it is dressed in overalls and looks like work."Thomas Edison

sxshep wrote:Not sure if someone mentioned this... but what category would Sig's "Ilaflon" coating fall under? I've tried doing research to see the differences between Ilaflon and regular bluing, but can't find much. My Sig Pro SP2022 has Ilaflon and I love how it looks

More marketing hype. This is just a tradename for a Teflon-based coating:Linky

There might be some sort of "ceramic" material embedded in the Teflon coating to help improve its longterm wear resistance, but that's about it. Think of your nonstick cookwear - they just put the same stuff on a gun.

Morne wrote:NOTE - Stainless steel parts that get EITHER carburized OR nitrocarburized are, metallurgically speaking, no longer stainless. The reason is because the carbon combines with the chromium to form chromium carbides.

Soooooo - then the few gunsmiths and such out there that're beadblasting M&P slides down to bare metal are doing folks a disservice?

Total repeal of ALL firearms/weapons laws at the local, state and federal levels. Period. Wipe the slate clean.

Morne wrote:NOTE - Stainless steel parts that get EITHER carburized OR nitrocarburized are, metallurgically speaking, no longer stainless. The reason is because the carbon combines with the chromium to form chromium carbides.

Soooooo - then the few gunsmiths and such out there that're beadblasting M&P slides down to bare metal are doing folks a disservice?

If people are happy with the work, then no. I wouldn't intentionally remove the case hardening from my slide.

Heck, my slide is my hammer for driving roll pins and punches on my M&P, pretty much all of the marks wipe right off. Why would I get rid of that feature on the hammer? Wait, did I say hammer? I meant slide....

I guess my question is whether they still have any appreciable corrosion resistance?

It's really dependant upon the media used, how long it was blasted, how close the nozzle was, and at what pressure. This determines how deeply they cut into the surface. If they cut below the hardened surface, it will probably have to keep oiled now and then. If they used say, glass, at 30psi, 12 inches away from the part, in broad sweeping passes, I highly doubt it would harm the corrosion resistance at all. Aluminum oxide at 120 psi, moving extremely slowly, 4 inches away from the part is a different ball park...

I guess my question is whether they still have any appreciable corrosion resistance?

Technically, blasting off the nitrocarburized layer from a stainless steel will IMPROVE its life expectancy in a corrosive environment.

Now the point about media is a good one. But provided that the blasting is aggressive enough to completely remove the case hardened layer then the only remaining issue is one of cleanliness of the media. Virgin grit will leave the stainless surface passivated. Obviously, following this with an acidic passivating treatment is advisable.

Different media will get you different results. NEVER blast stainless steel with iron/steel grit, unless followed by an acid passivating treatment to remove any embedded non-stainless pieces. Glass bead is better for a burnished sort of finish. Personally, I am partial to aluminum oxide.

There's a whole myriad of electroless nickel based emulsion coatings. They have snazzy names like "Niboron", "NP3", "Composite Diamond Coating" and so on. But what are these? Simply put, they are electroles nickel platings (see original post for more info) with other stuff sprinkled in. The "sprinkling" is done by emulsifiying the additive in the electroless nickel plating bath, which is different from dissolving it. As a result, the coating is the most consistent on simple geometry parts.

The great thing about these coatings is the ability to add another characteristic to what is otherwise a fairly good wear resistant coating. Adding teflon, like the "NP3" coating, improves lubricity. Adding Boron, like the "Niboron" or "FailZero" coatings, uses the Boron to increase hardness rather than the phosphorus. Adding nano-diamonds, like the "Compositve Diamond Coating", increases bulk hardness and wear resistance.

Other additives include silicon carbide, aluminum oxide, molybdenum disulfide (a classic dry film lubricant), glass and tungsten carbide. Not all of these coatings have found applications yet in the firearms industry, but they could.